This invention relates to novel, relatively low molecular weight polyalkylacrylate prepared under neat conditions which is compatible in lubricating oil concentrates with ethylene-propylene viscosity index (VI) improvers and which exhibits in finished lubricating oil compositions improved pour depressing effects. In addition, the invention is directed to lubricating oil compositions wherein the polyalkylacrylate pour depressant and ethylene-propylene viscosity index (VI) improver are compatible and to a method of producing such compositions.
Hereinbefore and hereinafter the terms "polyalkylacrylate" and "alkylacrylate" are intended to encompass the alkyl ester derivatives of methacrylic acid as well as acrylic acid, i.e., polyalkylmethacrylate, polyalkylacrylate, alkylmethacrylate and alkylacrylate. Further, the term "neat" describes the undiluted product as well as the manufacture condition in which no liquid diluent is employed. Still further, the term "compatible" signifies the ability to form and maintain for long periods of time (i.e. months) a single homogenous phase. It has been found the degree of compatiblity of the polyacrylate and ethylene-propylene polymer in lubricating oil compositions is a direct function of the degree of haze in the initially formed composition, e.g., as measured by Lumetron Turbidity (LT). Compatibility is essentially premanent when initial Lumetron Turbidities are less than about 10.
In automotive lubricating oils two of the most common types of additives found therein are VI improvers and pour point depressors. In brief explanation of VI, it refers to the rate of change of viscosity of lubricating oil. Oils having a high VI exhibit a small change in viscosity with temperature variation than those with low VI. In lubricating applications such as engine lubrication where a wide range of temperatures is often encountered, lubricating oils having a high VI are desirable and often critically necessary. Long chain polymers and copolymers are in many instances effective VI improvers. Unfortunately, high molecular weight polymers are often susceptible to high shear rates in lubricating service which can cause a viscosity loss and other undesirable effects in the oil blend. One VI improver additive which has proven effective as a viscosity improver while exhibiting a high degree of shear stability is an amorphous copolymer of ethylene and propylene having an average molecular weight between about 10,000 and 150,000, preferably between about 30,000 and 80,000, a propylene content of between about 20 and 70 mole % and a M.sub.w M.sub.n (polydispersity index) of less than about 5. This copolymer is described in U.S. Pat. Nos. 3,522,180 and 3,697,429.
The often used companion additive with the ethylene-propylene copolymer is the pour depressor polyalkylacrylate prepared from C.sub.1 -C.sub.22 alkylacrylate monomer, mixtures thereof, and mixtures of C.sub.1 -C.sub.22 alkylacrylate monomers and nitrogen containing compounds such as dialkylaminoalkyl methacrylates and n-acrylamides. The polyalkylacrylate polymers normally have a molecular weight between about 30,000 and 1,500,000 with the higher molecular weights normally favored on the basis of greater pour depressancy. However, one of the continuing problems with the polyacrylate pour depressor-ethylene-propylene VI improver combinations is their incompatibility under concentrate conditions, that is, under conditions the pour depressor constitutes about 1 and 10 wt. % and between about 5 and 30 wt. % VI improver in at least 50 wt. % (major amount) lube oil base. As heretofore stated, this incompatibility takes the form of layered phase separation of the two additives which results in inappropriateness for blending into the finished composition since phase separation in large holding tanks will tend to cause the non uniform addition of additives in the finished formulation. Concentrate lube formulation as opposed to finished formulation is prepared in such form for savings in storage and transportation and are formed into the finished composition by blending the additional base oil.
One solution to the problem is set forth in U.S. Pat. No. 3,897,353 wherein the molecular weight of the polyalkylacrylate is controlled to between 30,000 and 120,000. The polyalkylacrylates of the patent were found to be compatible with ethylene-propylene copolymers in concentrate formulations for extended periods of less than three months. However, for longer periods, e.g., 3 to 6 months incompatibility was still evidenced.
In another prior art disclosure, U.S. Pat. No. 3,892,671 teaches lubricating oil compositions containing a compatible combination of nitrogen containing polyalkylacrylate of a molecular weight of from 2000 to 20,000, preferably 6000 to 10,000 prepared under diluent conditions and combination ethylene-propylene VI improver. Apparently in U.S. Pat. No. 3,892,671 a required factor for compatibility appears to be the use of nitrogen moiety in the polyacrylate.
Although the pour depressancy and VI effect appeared to be satisfactory in the foregoing patents, there is a continuing need to extend the term of compatibility of the polyalkylacrylate ethylene-propylene copolymer combinations, particularly in lube concentrates in addition to improving pour depressancy, VI, etc. properties. Further, from an economic standpoint, it is desirable to improve compatibility without having to resort to the more costly nitrogen containing polymethacrylates.